1. Field of the Invention
This invention relates to a method of compensation for changes in the output of read-out signals caused by shading in an apparatus for reading out an image by scanning a light beam on a recording medium carrying the image recorded thereon, obtaining light which carries the image by the scanning, and detecting the light by a photomultiplier.
2. Description of the Prior Art
Image read-out apparatuses for reading out an image by scanning a light beam on a recording medium carrying the image recorded thereon and detecting light reflected by the recording medium, light passing through the recording medium, or light emitted by the recording medium 15 have heretofore been used as, for example, image input means for computers and image read-out means of facsimiles. Also, as one of the image read-out apparatuses of this type, there has heretofore been known a radiation image read-out apparatus as disclosed in, for example, U.S. Pat. No. 4,346,295 wherein a stimulable phosphor sheet carrying a radiation image of an object such as the human body stored thereon is scanned by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, and the light emitted by the sheet portion exposed to stimulating rays is detected to obtain image signals representing the radiation image.
As one of photodetectors used in the aforesaid image read-out apparatus, there has heretofore been known a long photomultiplier as disclosed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-16666. The long photomultiplier has a substantially long light receiving face which extends along a main scanning line on a recording medium. With the long photomultiplier, nonuniformity of the light receiving sensitivity arises in the longitudinal direction of the photomultiplier.
Also, the output of the photodetector may change due to nonuniformity of the intensity of the scanning light beam caused by nonuniformity of the reflectivity of a reflecting surface of a light deflector for deflecting the light beam in the main scanning direction, nonuniformity of the scanning speed of the light beam caused by fluctuations in the deflection speed of the light deflector, or light guiding nonuniformity of a light guide member provided for efficiently guiding the light obtained by the recording medium to the light receiving face of the photomultiplier, i.e. nonuniformity of the light guiding efficiency at, for example, an end portion of the photodetector. In the case where partial deterioration of the light detecting efficiency (i.e. shading) is caused by the aforesaid nonuniformities, it becomes impossible to accurately detect the light obtained by the recording medium.
In order to eliminate the aforesaid problems, there has been proposed a shading elimination method wherein shading characteristics in the direction of main scanning of a light beam are detected and stored prior to detection of light obtained by a recording medium, and read-out signals obtained by a photomultiplier are corrected in accordance with the shading characteristics at the time of the detection of the light so that output fluctuations caused by the shading are eliminated.
Basically, the proposed shading elimination method provides good results. However, the inventors found that nonuniformity characteristics of the light receiving sensitivity of the photomultiplier changes slightly when the high voltage applied to the photomultiplier is changed. In the case where nonuniformity characteristics of the light receiving sensitivity of the photomultiplier changes in this manner, it may occur that the shading characteristics detected in advance and the actual shading characteristics at the time image read-out processing is being carried out do not coincide with each other. As a result, shading elimination cannot be carried out accurately with respect to the actual shading characteristics.